Twist drill

ABSTRACT

The invention relates to a twist drill, especially for metals. It has a substantially cylindrical basic component (1) divided into a shaft (2) and a cutting section (3), the central longitudinal axis (5) of which is the axis of rotation of the twist drill. Several swarf grooves (4, 4&#39;) are made in the cylindrical surface of the basic component (1) which extend spirally and continuously from the shaft (2) to the drill tip (6) and bound a swarf chamber (35). The feature of the twist drill is that its drill core diameter tapers continuously from the drill tip (6) to the shaft (2).

The invention relates to a twist drill, especially for processingmetals. Such drills normally have a cylindrical basic component which isdivided into a shaft and a cutting section. Several swarf grooves aremade in the surface of the cutting section which extend spirally aroundthe central longitudinal axis of the drill or the drill core and end inthe o-face which forms the drill tip. Drills for a specified drillingdepth that is greater than or equal to three times the drill diameterare referred to as long drills. In other words, the minimum drillingdepth in long drills is three times the diameter of the drill.

A problem with long drills is that undesirable torsional vibrations mayoccur because of the ratio of drill length to drill diameter. A furtherproblem is chip removal with increasing drilling depth. During drilling,the chip which is removed in the effective range of the drill tip or thedrill's major cutting edges is rolled in the swarf groove between theface, which is the swarf groove surface, on the one hand, and theinterior wall of the bore hole, on the other hand, and is given aroughly spiral form. As the boring process proceeds, this chip orfragments thereof are transported in the swarf groove in the directionof the shaft end of the drill. The chip thereby robs against both theface and the interior wall of the bore hole. This friction decreases thechip removal rate and can finally cause the chips to back up. Otherchips being pushed along cause a further increase in the friction forceswhich in turn causes a substantial increase in the cutting forces andincreases heat development. This results in disproportionate wear of thedrill's major cutting edges. In the extreme case, a backing up of thechips can cause the drill to break.

Automatic drills are frequently linked to an electronic monitoringsystem. This type of electronic monitoring system would interpret adisproportionate increase in the cutting forces as an indication of veryhigh drill wear or chipping of the cutting edge and would stop thedrill. In fact, however, the drill edge would not show any wear at all.Instead, there would be a backup of chips which could not be relieved bysimply switching off the machine. For to eliminate the chip backup, thedrilling tool would need to continue to run without feed in order toremove the collected chip material from the bore hole through the swarfgroove.

Drilling could then resume normally.

A long drill known from DE-DA-39 27 615 solves the torsional vibrationproblem by expanding the drill core from the tip to the shaft. However,this impedes chip removal because the depth of the swarf groovedecreases with increasing distance from the drill tip. To remedy this,the swarf grooves of the known drill were substantially widened. This isexpressed in the ratio of back width or minor flank to groove widthwhich is 0.8-0.9. However, this widened swarf groove can improve chipremoval only to a limited extent because the depth of the swarf groovedecreases with increasing distance from the drill tip. A round or spiralchip which is formed in the effective range of the major cutting edgesof the drill and has a diameter corresponding to the swarf groove depthin the area of the drill tip would, with increasing distance from thedrill tip, have increasing difficulty to fit into the swarf chamberbounded by the face and the interior wall of the bore hole. As a result,the chip would increasingly rub against the face and the interior wallof the bore hole and thus cause the chip removal rate to decrease.

Also known in prior art are twist drills the core diameter of whichincreases towards the shaft while their angle of twist decreases toimprove chip removal in the same direction.

Based on the above, it is the object of the invention to make a twistdrill in such a way as to avoid the backup of chips and to make itpossible to operate the drill at high cutting rates over the entiredrilling depth or cutting length without any significant torsionalvibrations. To accomplish this, the invention takes an approach which isvery different from that of prior art. In contrast to prior art, thedrill core diameter according to the invention does not increase fromthe drill tip toward the shaft, instead it is tapered continuously atleast along a portion of the cutting component that is adjacent to thedrill tip. The taper of the core diameter necessarily implies that thedepth of the swarf groove, that is, the radial clearance between theinterior wall of the bore hole and the face area that touches the corediameter, increases. Although the increase in the swarf groove depthalso implies an increase in the cross-sectional area or the volume ofthe swarf groove, it is not the increased area or volume which arecrucial, but almost exclusively the increased swarf groove depth. Thediameter of the chip which is removed in the effective range of thedrill tip approximately corresponds to the depth of the swarf groove inthe tip area of the drill. While the chip rubs against both the face andthe interior wall of the bore hole in the tip area of the drill, it istransported almost exclusively by the face of the swarf groove and thuswith less friction in that area of the bore hole which is remote fromthe tip. Since friction along the interior wall of the bore hole is nowpractically absent, the force required to transport the chip is reduced.The reduced resistance to the chip's transport prevents or decreases thebraking action so that the chip removal rate remains constantpractically along the entire cutting length of the twist drill, therebyeffectively avoiding a backup of chips. Furthermore, reduced friction inthe transport of the chip decreases friction-induced heat development onthe face.

It was revealed that despite the taper of the core diameter in thedirection of the shaft of the twist drill, no significant torsionalvibrations occur. This can primarily be attributed to the reducedfriction in chip removal and the almost constant cutting force overpractically the entire drilling depth. Another advantage is that theconnection of the drill to an electronic monitoring system will notresult in interruptions of the drilling process which are due toincreased cutting forces because of chip backup.

As previously mentioned, the increase of the cross-sectional area or thevolume of the swarf groove is less crucial than the clearance betweenthe interior wall of the bore hole and the face, that is, the swarfgroove depth. Keeping constant the cross-sectional shape of the swarfgroove or the curvature of the face, also as seen in cross-section,makes it possible repeatedly to grind the twist drill according to theinvention. Particularly for high-quality drills, such as solid carbidedrills, this is a decisive cost advantage. It is well known thatregrinding a twist drill causes the drill or cutting length to beshortened due to the removal of material in the drill tip area. Awidening of the swarf groove for the sole purpose of increasing itscross-sectional area would not allow repeated grinding of a twist drill,because widening the swarf groove in the corresponding areas which areremote from the tip causes completely different swarf groove profiles orface curvatures which are unsuitable for making or forming chips. Bycontrast, the drill according to one embodiment 2 of the invention has aswarf groove profile or curvature which remains substantially constantalong the entire section that is usable for drilling, that is, along thecutting length of the drill's cutting component. As a result, after eachgrinding process, the shape of the face in the twist drill tip area isalways such as to be suitable for making and forming chips. By nature,this can only be achieved if the core diameter of the twist drillaccording to the invention is not abruptly but continuously tapered. Itis not necessary, however, that the continuous taper of the drill corediameter extend along the entire length of the cutting component. Withfrequently reground drills which retain only a portion of their originallength it is in any case no longer possible to obtain the greaterdrilling depths. Since chip removal is less of a problem with decreaseddrilling depth, it may be appropriate to dispense with a furtherdecrease in the drill core diameter in the area of the cutting componentwhich is close to the shaft. In any case, with very long drills,stability considerations limit the extent to which the drill corediameter can be decreased.

The face area which primarily affects chip formation is that portion ofthe face which has the smallest radius of curvature. According toanother embodiment of the invention, this radius of curvature,hereinafter referred to as chip forming radius, increases withincreasing distance from the drill tip. Again, it is not thecorresponding possible increase in the cross-sectional area of the swarfgroove that is crucial, but rather the following effect: when the chipis formed, the chip forming radius determines the degree of curvature ordiameter of the spiral-shaped chip. If the chip forming radius were toremain constant along the cutting length of the twist drill, the chipforming property of the face at a point which is remote from the tipwould no longer be optimal. For chip formation not only depends on theface, but also on the interior wall of the bore hole and particularly onthe ratio of the difference between drill diameter and drill corediameter and the chip forming radius. Since an increase in the distancefrom the drill tip implies an increase in swarf depth, i.e. the radialclearance between the face section which is crucial in chip formationand the interior wall of the bore hole and, respectively, the differencebetween the drill diameter and the drill core diameter, theaforementioned dimension ratio would, with a constant chip formingradius and increased distance from the drill tip, assume values that liefurther outside the optimum value range. The chip forming properties ofswarf groove areas that are remote from the tip, which would be theeffective range of the drill after repeated grinding, would continue todeteriorate.

According to a further embodiment of the invention, the above stateddimension ratio assumes a value of 4.0 as its greatest value. This valueis reached when the chip forming radius is half the swarf groove depth.According to the invention, the value of said dimension ratio can varybetween 4.0 and 2.7. The value of 2.7 is reached when the chip formingradius is approximately 3/4 of the swarf groove depth. Within theclaimed value range, satisfactory chip formation and chip removal areensured. It is advantageous to retain the selected dimension ratio alongthe cutting length or the length that may be reduced by grinding so thatafter each grinding process the same geometric conditions and thereforethe same chip forming properties exist in the effective range of thedrill tip. It is also feasible, however, that said ratio taper withincreasing distance from the drill tip. However, the taper should notexceed a maximum of 40% of the original value at the drill tip.

The core diameter of a twist drill according to the invention isselected in such a way that it lies within a range of 0.22 D-0.35 D,where D is the diameter of the drill or the cutting component. The value0.22 D is the minimum value for the drill core diameter. A thinner drillcore would risk breakage of the drill. On the other hand, the value 0.35D represents the lower limit of the swarf groove depth. A thicker drillcore would make the swarf groove depth too shallow to ensurelow-friction chip removal according to the invention. The taper of thecore diameter with increasing distance from the drill tip shouldpreferably be 0.2 mm-0.8 mm per 100 mm drill length. The twist drillaccording to the invention is preferably a solid carbide drill.

Thus, the design of the swarf groove according to the invention makes itpossible to regrind the twist drill within a very wide range. It is evenpossible that a frequently reground twist drill which has become tooshort to drill deep bore holes can continue to be used for shallowerboring depths. The continuous increase in the swarf groove depth andretention of the swarf groove cross-sectional shape over almost theentire cutting length makes it possible to continue to use the twistdrill according to the invention even if it has been considerablyshortened compared to its manufacturing length.

In addition, the twist drill design according to the invention resultsin a reduction of the cutting forces. This reduction in the cuttingforces has the direct effect of reducing stress on the cutting materialitself. In turn, these factors increase the life of the toolconsiderably. Tests have shown that tool life can be doubled or tripledwith a drill according to the invention. The improvement in economicefficiency and productivity that is connected with prolonged tool lifeis self-evident.

An embodiment of the twist drill according to the invention is describedbelow and the attached drawings are used to illustrate additionaladvantages of the drill according to the invention.

FIG. 1 shows a side elevation of the twist drill according to theinvention.

FIG. 2 shows a schematic side elevation of the drill according to FIG. 1with a schematic representation of the open drill core.

FIG. 3 shows a section based on Line III--III in FIG. 1.

FIG. 4 shows a section based on Line IV--IV in FIG. 1.

FIG. 5 shows a section along Line V--V in FIG. 1.

FIG. 6 shows a section along Line VI--VI in FIG. 1.

FIG. 7 is a representation in which the sections according to FIGS. 3through 5 are magnified and congruently superposed one on top of theother.

FIG. 8 shows a section according to FIG. 4 but includes a schematicallydrawn chip.

FIG. 9 shows a section according to FIG. 5, also including the chipinside the swarf groove.

FIG. 1 shows a twist drill (hereinafter simply referred to as drill)having a substantially cylindrical basic component 1 which is dividedinto a shaft 2 and a cutting component 3. Two diametrically opposedswarf grooves 4, 4' are made in the surface of the cutting component 3.The swarf grooves extend spirally around the central longitudinal axis 5of the drill and open onto the end face of the drill which forms thedrill tip 6. The cutting component 3 can be divided into two sections,one usable section which extends from the drill tip 6 to the extensionline 7 and which is the cutting length 8 of the cutting component 3.Between the section that defines the cutting length 8 and the shaft 2,there is an intermediate section where the swarf grooves 4, 4' level offand end in the surface of the cutting component 3. This intermediatesection forms the runout 11 of the cutting component 3. The drill'scentral longitudinal axis 5 is also its rotational axis around which itturns in the direction of the arrow 12 when it is in operation.

For the rest, the drill according to FIG. 1 has the usual features of atwist drill, such as two major cutting edges 13, 13', two major flanks14, 14', and two minor flanks 15, 15', each having a margin 16, 16'. Themajor cutting edges 13, 13' together with an assumed working plane 17form the side clearance angle α. The angle of twist of the swarf grooves4, 4' is preferably constant along the entire cutting length.

FIG. 2 shows the cutting component 3 and a cross section through thedrill core 18. The extension lines 21 which flank the drill core 18 markthe extension of the cutting component 3 in transverse direction 22.They represent the longitudinal section lines of an imaginary envelopehaving a diameter which corresponds to the diameter 23 of the cuttingsection 3. As FIG. 2 clearly shows, the drill core 18 continuouslytapers from the drill tip 6 towards the shaft 2. The drill core diameter24 is therefore greater in the area of the cutting component 3 near thetip than in the area close to the shaft. The taper of the drill core isselected such that the decrease in the drill core diameter 24 over alength of 100 mm is 0.2-0.8 mm. The length of the runout 11 of the drillis 1.5 times the drill diameter 23.

In the cross-sectional representations of FIGS. 3 through 5, thecircular line which defines the cross section of the cutting componentis the envelope line 26 of the cylindrical surface of the cuttingcomponent 3. As FIGS. 3 through 5 clearly show, the drill core diameter24 tapers from FIG. 5, which shows a section through the tip area of thedrill (Line V--V in FIG. 1), to FIG. 3, which shows a section throughthe area near the shaft of the cutting component 3 (Line III--III inFIG. 1). This decrease is connected with an increase in the swarf groovedepth 27. To promote clarity, the swarf groove depth 27 is indicatedonly for the one swarf groove 4 of the two swarf grooves 4, 4'. FIGS. 3through 5 and FIG. 6 show the curvature of the face 9, 9'. The faceextends inward, in convex and substantially radial manner, from thefront edge of the margin 16, 16' which forms the minor cutting edge 28,28' to the central longitudinal axis 5 and touches the drill core in thecenter of the cutting component 3. From there it extends outward again,with approximately the same curvature and in radial manner, and changesits curvature direction at a curvature reversal point 31. By nature, thearea of the face 9, 9' which extends radially outward from the curvaturereversal point 31 has practically no influence on the shaping of a chipremoved by the tool. This area is usually referred to as the backchamfer and is production engineering dependent.

The curvature of the face 9, 9' as seen in cross-section can be viewedapproximately as an arc section of an ellipse 32 (FIG. 6). Thelongitudinal axis 33 of the ellipse 32 is thereby approximately radiallyoriented to the drill's central longitudinal axis 5. However, it is alsofeasible that the curvature of the flank 9,9' more closely resembles acircular arc.

FIGS. 3 through 5 furthermore show that the main chip forming area 34,which is adjacent to the drill core 18 and primarily defines theformation and especially the curvature radius of the chip, has acurvature radius, hereinafter referred to as chip forming radius 35,which increases from FIG. 5 to FIG. 3. Thus, the radius of the face 9,9'which defines chip formation is the chip forming radius 35. The chipforming radius 35 decreases with increasing distance from the drill tip6. In other words, the curvature of the main chip forming area 34flattens out visibly.

FIG. 7 shows the magnified cross sections of FIGS. 3 through 5congruently superposed one on top of the other. The individual curvesare identified as III, IV, and V according to the section lines inFIG. 1. This representation again clearly shows the taper of the drillcore diameter 24. It also shows that in addition to the increase in theswarf groove depth 27 and the resulting increase in the cross-sectionalarea of the swarf chamber 36, there is an additional increase in theswarf chamber 36. It is due to the fact that the face area which isadjacent to the margin 16,16' curves increasingly inward with increasingdistance from the drill tip 6. The increase in the resultingcross-sectional area 37 corresponds to the area segment which is boundedby the cross-sectional lines III and V, is approximately sickle-shaped,and extends from the margin 16,16' to approximately the drill core 18.However, this area increase is of minor significance in the twist drillaccording to the invention, as will be explained in more detail below.Finally, FIG. 7 shows how the shape of the back chamfer, that is, theface area which extends radially outward from the curvature reversalpoint 31, changes with increasing distance from the drill tip. Thischange is substantially production-engineering dependent and does notcrucially affect chip removal or the chip-forming behavior of thepresent drill. However, such widening facilitates the supply of coolingliquid. Due to the change in the shape of the back chamfer withincreasing distance from the drill tip 6, there is an increasingwidening of the swarf groove 4,4'. This widening is also evident in FIG.1 which depicts the swarf groove width 41 measured at an angle to thespiral direction 42 of the swarf groove 4,4'. The swarf groove width 41increases in the direction of the shaft 2 such that the swarf grooves 4and 4' which are adjacent in the plan view of FIG. 1 differ in theirswarf groove width 41. The swarf groove area closer to the shaft has agreater swarf groove width 41.

FIGS. 8 and 9 are used to illustrate the operation of the drillaccording to the invention: FIG. 8 schematically shows chip formation inthe effective range of the drill tip 6, that is, in an areaapproximately coinciding with the section line V--V of FIG. 1. As thedrill turns in the direction of arrow 12, a chip 44 is removed by themajor cutting edges 13, 13' which in FIG. 8 run approximately in thedirection of the center line 43. With continued rotation of the drill,this chip moves along the face 9, 9' in the direction of arrow 12 and,due to the curvature of the face 9, 9', is deflected in the direction ofarrow 45 and spirally deformed. The area of the face 9, 9' which definesthe outer curve of the chip 44 is the main chip forming area 34 whichadjoins the drill core 18. This area has the smallest curvature radiusof the entire face 9, 9', namely the chip forming radius 35, andtherefore deflects the chip 44 most strongly. The interior circumferenceof the chip 44 is compressed as is indicated by the compression lines 46in FIGS. 8 and 9. After the chip 44 reaches a certain length, itfrequently breaks, preferably along the compression lines 46. Withadvancing drilling depth, the chip is transported from the effectivepoint of the drill, which is its tip area, in the direction of thedrill's shaft 2. In these areas of the swarf groove 9,9' or the swarfchamber 36 which are remote from the tip, the drill core diameter 24 issmaller and the swarf depth 27 correspondingly greater. Due to thecurvature of the main chip forming area 34 which is present in the tiparea according to FIG. 8, the chip 44 receives a corresponding curvatureand thereby a chip diameter 47 which approximately corresponds to suchcurvature. With increasing distance from the drill tip 6, the differencebetween the swarf groove depth 27 and the chip diameter 47 noticeablyincreases. As a result, the chip or chip fragments in the areas remotefrom the tip no longer lie or rub against the interior wall of the borehole which approximately corresponds to envelope line 26 in FIGS. 8 and9. The guiding surface instead is only the face area which remains incontact with the outside circumference of the chip 44. In the swarfgroove areas which are remote from the tip, the chip 44 is thereforeguided practically only by said face areas which are in contact with theoutside circumference of the chip 44 (see FIG. 9). As a result, theresistance to chip removal decreases as the distance from the pointwhere the chip is formed increases. This effectively prevents a chipbackup and the well-known concurrent phenomena which are an increase inthe cutting force and in the temperature of both the workpiece and thedrill.

A drill according to the invention can be repeatedly ground because thecross-sectional shape of the swarf groove 4, 4' or the swarf chamber 36corresponds to the original tip area along almost the entire cuttinglength 8. For example, grinding the drill to a residual lengthcorresponding to Line IV--IV of FIG. 1, results in a cross-sectionalshape that approximately corresponds to that depicted in FIG. 9. In thiscase, however, the formed chip 44 would also be in contact with the wailof the bore hole (zero line 26) as depicted in FIG. 8. For the chipforming radius 35 which defines the shape properties of the main chipforming area 34 increases in the direction of the shaft 2 so that thedeveloping chip 44 has a correspondingly greater outer curvature andthereby a correspondingly greater chip diameter 47. This diameter 47would then approximately correspond to swarf groove 27 in FIG. 9.

The difference of the drill diameter 23 (D) and the drill core diameter24 is twice the depth of the swarf groove 27. The swarf groove thereforeis D-D_(K) /2. In order for a chip 44 to fit into a swarf chamber 36having a predetermined swarf groove depth 27, the chip forming radius 35must be approximately half of the swarf groove depth or D-D_(K) /4.Smaller chip-forming radii 35 (R_(F)) are undesirable because they nolonger ensure the combined action of the interior wall of the bore holeand of the face 9,9'. However, R_(F) can assume greater values. Theupper limit determined for a chip forming radius R_(F) max. is D-D_(K)/2.7.

Testing was conducted to show the dependence of the cutting force andthe torque of a twist drill as a function of the drilling depth. Thetest conditions for a drill according to prior art (Tool 1) and theinvention (Tool 2) are shown below. It is found that for the prior arttool that there is a gradual increase in the cutting force and thecorrelated torque starting with a drilling depth of approximately 40 mmand an abrupt increase at a drilling depth of approximately 45 mm.

It is found that both the cutting force and the torque are clearly belowthe comparison values of prior art tool 1 and that there is no increaseof these parameters at greater drilling depths. The result is a constantcutting force and a constant torque across practically the entirecutting length or drilling depth.

The following table lists the test conditions and the various materialsused.

    ______________________________________                                        Workpiece:                                                                            Wheel support for an automobile                                       Material 45 M5 UA2.sup.1)                                                                            Hardness: 269HB.sup.2)                                 Bore hole diameter 11 mm                                                                             Drilling depth: 55 mm                                  Machine:                                                                              Machining center with coolant supply by means of a                            spindle, coolant pressure 18 bar                                      Cutting data:                                                                 Rotational speed:      2315 1/min                                             Cutting speed:           80 m/min                                             Feed rate:              695 mm/min                                            Feed rate per revolution:                                                                              0.3 mm                                               Tool 1: Solid carbide drill with cooling duct having 4 margins and                    a conventional swarf groove geometry, that is, constant                       core diameter and constant swarf groove profile from drill                    tip to swarf groove runout.                                           Cuttin, material:      PVD.sup.3 coated carbide                                                      metal for P40                                                                 application                                            Drill tip sharpening according to EP0249104A1                                 Core diameter:         3.2 mm                                                 Chip forming radius:   2.7 mm                                                 Ratio (D-Dk)/Rs:       2.9                                                    ______________________________________                                         .sup.1) French material standard AVNOR                                        .sup.2) Brinell hardness                                                      .sup.3) Plasma vapor deposition                                          

    ______________________________________                                        Tool 2:                                                                              Solid carbide drill with cooling duct having 4 phases and a                   swarf groove geometry according to the invention                       Cutting material:      PVD.sup.3 coated carbide                                                      metal for P40                                                                 application                                            Drill tip sharpening according to EP0249104A1                                 Core diameter at drill tip:                                                                          3.2 mm                                                 Core diameter at swarf chamber runout                                                                3.0 mm                                                 Chip forming radius at drill tip:                                                                    2.0 mm                                                 Chip forming radius at swarf chamber runout                                                          2.5 mm                                                 Ratio (D-Dk)/Rs at drill tip:                                                                        3.9                                                    Ratio (D-Dk)/Rs at swarf chamber runout:                                                             3.2                                                    ______________________________________                                    

We claim:
 1. A twist drill, especially for metals, havinga substantiallycylindrical basic component (1) which is divided into a shaft (2) and acutting component (3), the central axis (5) of which is the rotationalaxis of the twist drill, and several swarf grooves 4, 4' which are madein the cylindrical surface of the cutting component (3) and extendspirally and continuously from the shaft (2) to the drill tip (6) andform a swarf chamber (36), characterized by the fact that the drill corediameter of the twist drill continuously tapers from the drill tip (6)to the shaft (2) at least in a partial section which is adjacent to thedrill tip, that at least a partial section extending from the drill tip(6) towards the shaft (2) of the section of the cutting component (3)which is usable for drilling and defines the cutting length (8) of thedrill has a substantially constant cross-sectional shape of the swarfgroove 4, 4' and, respectively, a curvature of the face 9, 9' whichforms the interior wall of the swarf groove, such that a main chipforming area (34) of the face 9, 9' which defines chip formation has achip forming radius (35) which increases with increasing distance fromthe drill tip (6).
 2. The twist drill according to claim 1 furthercharacterized by the fact that the ratio (D-D_(K)):R_(S) has a valuebetween 2.7 and 4.0, where D is the drill diameter (23), D_(K) the drillcore diameter (24), and R_(S) the chip forming radius.
 3. The twistdrill according to claim 2 further characterized by the fact that theratio (D-D_(K)):R_(S) is constant at least along a section of thecutting length
 8. 4. The twist drill according to claim 2 furthercharacterized by the fact that the ratio (D-D_(K)):R_(S) decreases alongthe cutting length (8) by a maximum of 40% with respect to the value inthe area of the drill tip (6).
 5. The twist drill according to claim 1further characterized by the fact that the drill core diameter (24) liesin the range of 0.22 D to 0.35 D.
 6. The twist drill according to claim1 further characterized by the fact that the taper of the drill corediameter (24) is 0.2 mm to 0.8 mm per 100 mm of drill length.
 7. Thetwist drill according to claim 1 further characterized by the fact thatit is a solid carbide drill.
 8. The twist drill according to claim 2further characterized by the fact that the drill core diameter (24) liesin the range of 0.22 D to 0.35 D.
 9. The twist drill according to claim3 further characterized by the fact that the drill core diameter (24)lies in the range of 0.22 D to 0.35 D.
 10. The twist drill according toclaim 4 further characterized by the fact that the drill core diameter(24) lies in the range of 0.22 D to 0.35 D.
 11. The twist drillaccording to claim 2 further characterized by the fact that the taper ofthe drill core diameter (24) is 0.2 mm to 0.8 mm per 100 mm of drilllength.
 12. The twist drill according to claim 3 further characterizedby the fact that the taper of the drill core diameter (24) is 0.2 mm to0.8 mm per 100 mm of drill length.
 13. The twist drill according toclaim 4 further characterized by the fact that the taper of the drillcore diameter (24) is 0.2 mm to 0.8 mm per 100 mm of drill length. 14.The twist drill according to claim 5 further characterized by the factthat the taper of the drill core diameter (24) is 0.2 mm to 0.8 mm per100 mm of drill length.
 15. The twist drill according to claim 2 furthercharacterized by the fact that it is a solid carbide drill.
 16. Thetwist drill according to claim 3 further characterized by the fact thatit is a solid carbide drill.
 17. The twist drill according to claim 4further characterized by the fact that it is a solid carbide drill. 18.The twist drill according to claim 5 further characterized by the factthat it is a solid carbide drill.
 19. The twist drill according to claim6 further characterized by the fact that it is a solid carbide drill.